Sympathetic activity and the systemic circulation in the spinal cat.

In the spinal animal, some sympathetic preganglionic neurones show background discharge and some can be reflexly excited or inhibited. Recent work has pointed out that while only a fraction of the total sympathetic outflow is involved, elements responsive to reflexes are present in each of many segments (Beacham & Perl, 1964a, b). The fact that only a limited number of the cells in each segment participate in a reflex could reflect differences either in the excitability of various preganglionic neurones or some other feature of organization. As a step toward further understanding of the role of the spinal cord in sympathetic mechanisms, the present study aims to establish effector organs controlled by preganglionic neurones active in the spinal preparation. Our attention was directed to the circulatory system because several investigators have described cardiovascular responses in spinal animals (Sherrington, 1906; Langley, 1924; Brooks, 1933; Alexander, 1945; Mukherjee, 1957). Difficulty in recording from sympathetic preganglionic and post-ganglionic cells simultaneously forced the use of an indirect approach. In most experiments the effects of nerve volleys or adequate stimuli known to initiate spinal preganglionic reflexes were tested on postganglionic neuronal activity, arterial pressure, blood flow or heart rate. Alternatively, preganglionic discharge and cardiovascular events were recorded in parallel. The results indicate that not all effector organs are influenced by sympathetic reflexes or autogenous activity in the spinal cat. In addition, cyclic or periodic changes in preganglionic discharge and in the systemic arterial pressure were noted which could be modified so as to suggest some form of sympathetic control of the circulation at the spinal level.